Within the electric power technique it is of great interest to know the temperature of current carrying apparatus and busbars. This can be used for controlling the magnitude of the load (e.g. in case of temporary overload) and for monitoring undesired power development in busbar joints, contacts and cable connections, whereby, for example, increased resistances caused by corrosion may be detected.
What makes the above measuring problem especially difficult is that the measuring points are positioned at high potential in connection with large current. The measuring system must handle high electric and magnetic fields and must not increase the risk of creeping currents or flashover. Preferably, it should be designed such that no new type tests need to be made to verify the safety.
In the publication Siemens-Energietechnik 6 (1984) No. 6, pp. 277-279, Jez et al: "Lichtleiter-Trennverstarker, ein Gerat zum Messen auf hohem Potential", a measuring device intended, inter alia, for temperature measurement in high-voltage plants is described. A considerable drawback in this device is that the current supply to the measured data transmitter takes place with the aid of an accumulator placed at high potential near the transmitter, which results in the high-voltage part of the equipment becoming bulky and maintenance-demanding. As temperature sensors in this equipment there are used thermocouples, which results in a weak analogue signal which requires amplification and conversion. For transmission of measured information from the transmitter at high potential to the receiver at ground potential, a light guide with optical fibres is used. This may in certain cases entail an increased risk of creeping currents and flashover.
The above-mentioned drawbacks can only be partly avoided by a device for temperature measurement described in patent specification EP-A-0263233. This device comprises a temperature sensor in the form of a quartz crystal. The crystal constitutes the frequency-determining element in an oscillator circuit, the output signal of which is converted into a sequence of pulses which are sent out as light to a detector. The temperature of the sensor is obtained by measuring the period of the received signal. The current supply of the sensor unit is performed, also in this device, with the aid of a battery or from an evaluation unit via a cable. Such a solution is therefore hardly applicable to temperature measurements at high potential. In addition, the device is relatively sensitive to disturbances from ambient light, since each period of the signal on which the measurement is based contains only one pulse. The evaluation unit of the device is intended to receive data from only one transmitter, which means that in the case of applications where temperature measurements are to be carried out at a plurality of measuring points, measuring equipment designed in this way will become relatively costly.
From patent specification DE-B-1904853 it is also known to use, in connection with temperature measurement in oil-filled transformers, a quartz crystal serving as a temperature sensor for control of an oscillator. The transmission of measured information to ground potential is in this case performed with the aid of ultrasonics. Neither in this device is it possible for one and the same receiver to receive and distinguish measured data from several transmitters.